1. Field of the Invention
The present invention relates to a method for manufacturing a variable-throat exhaust turbocharger of an internal combustion engine. The turbocharger has a nozzle throat area varying mechanism (hereinafter called variable-throat mechanism) to vary the blade angle of a plurality of nozzle vanes rotatably supported by members in a turbine casing. The present invention also relates to a method for manufacturing constituent members of the variable-throat mechanism.
2. Description of the Related Art
In the field of exhaust turbochargers of internal combustion engines, particularly small ones used for internal combustion engines for vehicles, an exhaust turbocharger is disclosed in Japanese Laid-Open Patent Application No. 59-78983 (hereafter referred to as patent literature 1). The turbocharger is composed such that an exhaust turbine made of ceramic material which operates under high temperature and a compressor made of metal which operates under low temperature are connected by brazing welding.
In this exhaust turbocharger, the end portion of the shaft part of the ceramic turbine rotor is tapered, and a tapered hole is provided in the metal compressor wheel to be engaged tightly with the tapered portion of the turbine rotor. Depressed portions are provided on the surface of one or both of the tapered shaft portion and tapered hole so that when the tapered shaft portion is connected with the tapered hole, a medium of solder will fill the depressions. A highly accurate connection between the ceramic material and metal material is thereby realized, and the rigidity of connection between the ceramic turbine rotor and metal compressor wheel is increased by the mediacy of the solder.
In the field of exhaust turbochargers, particularly small ones used for an internal combustion engines for vehicles, a variable-throat exhaust turbocharger is equipped with a variable-throat turbine with which the flow rate of exhaust gas entering the turbine rotor through the scroll of the turbine casing can be varied. The rate is varied in accordance with operating conditions of the engine such that the exhaust gas flow matches the optimal operation condition of the turbocharger. The use of variable-throat turbochargers has spread widely in recent years.
In such a variable-throat turbocharger, among the constituent members of the variable-throat mechanism which vary the blade angle of a plurality of the nozzle vanes, drive pins are connected to a drive ring. The drive ring is engaged with a lever plate connected to the nozzle vanes by a plurality of nozzle shafts. A control sleeve engaged with the drive ring is connected to a lever linked to an actuator. These connections between the parts are usually made in such a way that the drive pins and the control sleeve, which are shaft members, are fixed to the drive ring and lever, which are members having a hole or holes, by means of welding or caulking.
There has been proposed in the patent literature 1, as mentioned above, a method of connecting a ceramic turbine rotor to a compressor wheel made of metal by brazing welding. However, as a method of connecting constituent members of an exhaust turbocharger, this connecting method has problems as described hereunder. Therefore, the method can not be applied to connecting constituent members of a variable-throat mechanism.
The method of connecting by brazing welding proposed in patent literature 1 is applied to connecting a compressor wheel to a turbine rotor to which high torque is loaded at high temperature. Depressions are provided on the surface of the tapered portion of the turbine rotor shaft and on the surface of the tapered hole of the compressor wheel to fill solder therein. If solder is filled imperfectly in the depressions, fracture may occur. Therefore, in the aforementioned method of connecting the turbine rotor to the compressor wheel, it is necessary for the brazing welding in the range of the depressions to be uniform and reliable, and it takes a good deal of processing hours to connect the turbine rotor to the compressor wheel by brazing welding.
Further, in connecting the turbine rotor to the compressor wheel, it is difficult to confirm infiltration and spreading of the solder to the far side range of the depressions toward the bottom of the hole in the compressor wheel. Consequently, it is difficult to detect defects in the connection, and stable quality of the turbocharger can not be maintained.
Therefore, the above discussed method of connecting by the brazing welding means of patent literature 1 is not appropriate for connecting constituent members of a variable-throat mechanism.